Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

CFF92 - Star Test

  • Please log in to reply
58 replies to this topic

#1 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 09 October 2016 - 08:24 PM

I had not planned to do this as much of my optical bench is still in storage from a recently executed family move from rural south-central PA back to the Baltimore area, but there was some replies (trolls?) to my original post about my new CFF92.  

 

We are nearing the end of a massive renovation and much of my optical bench is still in storage...   I did have a tripod, a camera and an artificial star in the new house - as well as the CFF92.  Here are the results.  Spherical correction quite good, like close to 1/10 wave.  There may be some very slight de-centering or perhaps a very slight wedge issue, as I see subtle brightening of the of the left portion of the outer Fresnal ring.

 

For the images below, inside of focus is on the left; outside of focus on the right.  First set of images is closer to focus, second set in slightly further away from best focus.  Images were taken with a monochromatic (black/white) camera using a green filter - Wratten 56

Attached Thumbnails

  • CFF Startest 2.jpg
  • CFF Startest 1.jpg

  • Jared, Scott99, MooEy and 1 other like this

#2 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 18417
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 09 October 2016 - 08:33 PM

It sure don't look the same.



#3 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 09 October 2016 - 08:50 PM

It sure don't look the same.

 

For a moment, I hesitated responding...  

 

But, no - no optic does look exactly the same.  I think you need to understand how aberrations affect the star test especially on short focal length APO's (and cassegrains). 

 

Tell me what you see as it relates to optical abberations...    "Its sure don't look the same"  really does not say much.   I can tell you with some certainty that the Spherical correction is quite good.

 

Chas - please go download Abberator and model what 1/4, 1/8, 1/10 wave errors look like.  You will be astounded at how different the out of focus patterns are for even 1/8 wave optics.   

 

I believe that most of your experience is in reflective optics where Chromatism and Sphero-chomatic abberation plays no role.  


Edited by peleuba, 09 October 2016 - 08:51 PM.

  • Phil Cowell and Asbytec like this

#4 tonyt

tonyt

    Surveyor 1

  • -----
  • Posts: 1952
  • Joined: 01 Sep 2009
  • Loc: Australia

Posted 09 October 2016 - 09:49 PM

It sure don't look the same.

It looks pretty good. It's common for the intra and extra focal patterns to differ in ED and Apo refractors.

In particular the outer ring is very similar on both sides of focus.



#5 ron scarboro

ron scarboro

    Messenger

  • *****
  • Posts: 455
  • Joined: 03 Jan 2016
  • Loc: Raleigh, NC

Posted 09 October 2016 - 10:24 PM

I would say....

 

Is there any color to Vega in focus?

Are images sharp?

 

If so, you have what you thought you bought.

 

CFF makes fabulous scopes IMHO (caveat:  Never owed one, but have looked through two).  Paul is a talented optician and I think you can't go wrong with one of his scopes.

 

Knowing the optician is important to me.  Whether it by Yuri, Roland or Paul that puts the scope over the Chinese manufacturers.  

 

Enjoy your scope.  I'm sure it is fantastic.

 

Clear skies,

 

 

 

Ron



#6 akulapanam

akulapanam

    Mercury-Atlas

  • *****
  • Posts: 2551
  • Joined: 26 Aug 2012

Posted 09 October 2016 - 11:08 PM

That looks fantastic compared to my Star WO71



#7 dedo

dedo

    Mariner 2

  • -----
  • Posts: 245
  • Joined: 04 Jun 2010
  • Loc: Italy, Rome

Posted 10 October 2016 - 01:43 AM

It's really, really good!

If I'm not mistaken it should be possible to tweak the wedge on CFF scopes. Enjoy it under the stars now.



#8 Erik Bakker

Erik Bakker

    Cosmos

  • *****
  • Moderators
  • Posts: 7992
  • Joined: 10 Aug 2006
  • Loc: Netherlands, Europe

Posted 10 October 2016 - 02:01 AM

Thanks for posting these images Paul!

 

Like you, I see several asymmetries in the defocused star images.

 

Long distance shipping can do awful things to optics and may be the cause of the wedge/decentering issue you describe. But of course not the different into-extra focus images.

 

F/6 optics are much more difficult to manufacture to perfection than f/8 or even slower optics. And triplets are more sensitive to decentering than doublets. We do pay a price for very compact scopes that have little chromatic aberration.

 

If one of my high end scopes had the startest shown in your pictures, I would take action to at least cure the decentering/wedge issue, which may even show on axis coma at very high magnifications at the point of best focus.

 

The level of spherical aberration one can live with is a very personal thing. Let me just say I'm a sensitive guy  :lol:

 

Under the stars, all the aberrations in an optical system add up to the final star image a scope produces. Overall, the startest images of your CFF92 do not reach near 1/10th wave P-V quality in my opinion.

The aberrations in this scope likely give me a feel they that have a similar combined impact on the overall quality as a scope I once had that combined perfect correction in SA with astigmatism. Overall quality in that scope struggled to reach 1/6th wave P-V. And it showed under careful evaluation under the stars and visibly lowered the contrast of fine planetary detail.


  • mgwhittle likes this

#9 CHASLX200

CHASLX200

    Hubble

  • *****
  • Posts: 18417
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 10 October 2016 - 05:23 AM

 

It sure don't look the same.

 

For a moment, I hesitated responding...  

 

But, no - no optic does look exactly the same.  I think you need to understand how aberrations affect the star test especially on short focal length APO's (and cassegrains). 

 

Tell me what you see as it relates to optical abberations...    "Its sure don't look the same"  really does not say much.   I can tell you with some certainty that the Spherical correction is quite good.

 

Chas - please go download Abberator and model what 1/4, 1/8, 1/10 wave errors look like.  You will be astounded at how different the out of focus patterns are for even 1/8 wave optics.   

 

I believe that most of your experience is in reflective optics where Chromatism and Sphero-chomatic abberation plays no role.  

 

I have owned over 200 scopes and never had one yet that was 100% the same. I don't even use a star test to test optics. Give me Jupiter on my best nites at 500 to 1100x and that tells me all i need to know. And trust me i have the best seeing in the country on avg.

 

I would say i have owned around 100 Newts and 50/50 APO's ACHROS- SCT's-MAKS.



#10 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 October 2016 - 07:53 AM

 

If one of my high end scopes had the startest shown in your pictures, I would take action to at least cure the decentering/wedge issue, which may even show on axis coma at very high magnifications at the point of best focus.

 

The level of spherical aberration one can live with is a very personal thing. Let me just say I'm a sensitive guy  :lol:

 

Under the stars, all the aberrations in an optical system add up to the final star image a scope produces. Overall, the startest images of your CFF92 do not reach near 1/10th wave P-V quality in my opinion.

 

 

While I appreciate the comments vis-a-vis decentering/wedge/alignment.  I am not sure where the issue really is.  It may be in my imaging train.  I hastily set the scope up on a tripod to capture these images.  My test bench is still in storage.

 

My opinion of this is different then yours - the intra-extra focus images are not hugely different. Go compare 1/4 optics to 1/8 wave optics using modeling software and you will see what I mean.  Actually, let me do it for you...   See below.  I've modeled intra/extra focal 3rd (lower) order SA starting at 1/5 wave, 1/7, 1/10, 1/12, 1/20.  High order SA has relatively little impact on the image at best focus in cassegrains and triplet APO's  

 

In any event, I will repeat what I said previously - the sphercial correction is quite good.  I need to investigate the decentering/wedge/collimation.

 

aberrator 002.jpg



#11 junomike

junomike

    Hubble

  • *****
  • Moderators
  • Posts: 16971
  • Joined: 07 Sep 2009
  • Loc: Ontario

Posted 10 October 2016 - 08:06 AM

When I evaluate a telescope and find it's optics are 1/4th or better, I rarely do anything further unless I see an issue under normal viewing (not star testing at 300+)

Most telescopes I've owned were 1/6th or better and  I find that to be quite satisfactory for the most part.

Do some more testing (as it's always fun) but for the most part, get out and enjoy that beautiful Apo.

 

Mike


  • peleuba, Phil Cowell, n2068dd and 1 other like this

#12 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • Posts: 5976
  • Joined: 30 Dec 2006

Posted 10 October 2016 - 09:34 AM

Thanks for posting this.  What I don't see is bad or even moderate astigmatism or SA.  I do see what may be a mild zone in the middle based upon the differences in brightness at the center for the pictures close to focus.  I've seen that in a lot of objectives, even from the likes of AP.  I also see the slight difference in brightness you mention in the outer ring.  That might be a touch of coma.

 

Of course the best image would be the one at focus, especially if you can show the first diffraction ring.   Also, please describe in words what you saw as you slowly went down and through focus.  How did the pattern "behave"?

 

But that also brings up some important information about how you took the pictures, like what was the magnification and did you stack frames or are these individual frames.  High magnification is very important to help resolve optical errors that can hide under low magnification.  So my "observations" are predicated on the assumptions that high magnification and stacking were both employed to make the final images.

 

Jeff 



#13 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 October 2016 - 10:05 AM

Thanks for posting this.  What I don't see is bad or even moderate astigmatism or SA.  I do see what may be a mild zone in the middle based upon the differences in brightness at the center for the pictures close to focus.  I've seen that in a lot of objectives, even from the likes of AP.  I also see the slight difference in brightness you mention in the outer ring.  That might be a touch of coma.

 

Of course the best image would be the one at focus, especially if you can show the first diffraction ring.   Also, please describe in words what you saw as you slowly went down and through focus.  How did the pattern "behave"?

 

But that also brings up some important information about how you took the pictures, like what was the magnification and did you stack frames or are these individual frames.  High magnification is very important to help resolve optical errors that can hide under low magnification.  So my "observations" are predicated on the assumptions that high magnification and stacking were both employed to make the final images.

 

Jeff 

 

Glad to post it.  Not my first rodeo testing/evaluating optics in telescopes.   You are correct, I barely see any trace of astigmatism and spherical correction is really quite good for such a fast lens.  Agree on the slight brightening of the outer ring - coma or decentering etc.  But its at a very low level.  Also a mild hill in center of lens, but its below 1/8 wave and concentrated in middle so has virtually no impact on overall correction. 

 

No images were stacked - its a single frame grab from an AVI from a DMK camera using a green filter.  There was no need to stack frames as this was an indoor artificial star test in a controlled environment.  Magnification is a squirrely thing when taking images as its not expressed in power but rather focal length or focal ratio of the telescope as there is no eyepiece to magnify the images.

 

Disagree with stacking.  No need to stack if S/N ratio is already at an acceptable level.  If performing a Roddier analysis using a webcam, stacking would be helpful.  But this was a quick test in a controlled environment where local and astronomical seeing played no part.  This is exactly what I saw when placing an eyepiece in the focuser at aprox 200x.   


Edited by peleuba, 10 October 2016 - 10:08 AM.

  • Paul G likes this

#14 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • Posts: 5976
  • Joined: 30 Dec 2006

Posted 10 October 2016 - 10:39 AM

Thanks again Paul.  Good stuff.  I only mentioned stacking as fluctuations in air currents, even indoors, can be averaged out that way.  But I do now see that subtle brightening in both sets of images so it's not an issue.  

 

"This is exactly what I saw when placing an eyepiece in the focuser at aprox 200x".  BINGO!  that's what really matters.  Looks like a good lens IMO but I have to ask you, what you saw visually at focus.  Did the first ring go all the way around? 

 

Can you describe your test set up?  Pictures?  Very curious as I also love testing stuff.

 

Thanks

 

Jeff



#15 Derek Wong

Derek Wong

    Apollo

  • *****
  • Posts: 1265
  • Joined: 27 Apr 2008
  • Loc: Glendale, CA

Posted 10 October 2016 - 12:21 PM

 

 

If one of my high end scopes had the startest shown in your pictures, I would take action to at least cure the decentering/wedge issue, which may even show on axis coma at very high magnifications at the point of best focus.

 

The level of spherical aberration one can live with is a very personal thing. Let me just say I'm a sensitive guy  :lol:

 

Under the stars, all the aberrations in an optical system add up to the final star image a scope produces. Overall, the startest images of your CFF92 do not reach near 1/10th wave P-V quality in my opinion.

 

 

While I appreciate the comments vis-a-vis decentering/wedge/alignment.  I am not sure where the issue really is.  It may be in my imaging train.  I hastily set the scope up on a tripod to capture these images.  My test bench is still in storage.

 

My opinion of this is different then yours - the intra-extra focus images are not hugely different. Go compare 1/4 optics to 1/8 wave optics using modeling software and you will see what I mean.  Actually, let me do it for you...   See below.  I've modeled intra/extra focal 3rd (lower) order SA starting at 1/5 wave, 1/7, 1/10, 1/12, 1/20.  High order SA has relatively little impact on the image at best focus in cassegrains and triplet APO's  

 

In any event, I will repeat what I said previously - the sphercial correction is quite good.  I need to investigate the decentering/wedge/collimation.

 

attachicon.gifaberrator 002.jpg

 

This is my point about star testing.  It can give an excellent qualitative assessment of an optic along with the in focus images.  However, even two very knowledgeable and respected people can have a reasonable amount of disagreement on the amount of spherical aberration.  This is especially true when you mix other aberrations, including small decentering errors, plus a small amount of zonal or high order spherical that is likely present.  Aberrator is nice, but both it and Suiter are limited by display or printing technology (Suiter said this himself but I can't find the reference).

 

Before someone chimes in about using an obstruction, see here:

https://groups.googl...vE/H2bFIbY1WlkJ

 

Hopefully Paul will get his bench up and running again, but even so there are differences between test equipment that are illustrated by the differences between multiple samples of Mr. Rohr's testing and Airylab of the same scope.

 

My guess is that the in focus images of this scope would be quite good, with only the slightest differences from a longer scope visible at high powers in good to excellent seeing.  I am fairly picky (not as much as some of you) but would be happy with this scope for its intended purpose.

 

Derek


Edited by Derek Wong, 10 October 2016 - 12:40 PM.

  • BillP likes this

#16 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 October 2016 - 12:57 PM

Funny you mention fluctuating air currents...    This was in my basement and I had to turn off the dehumidifier because the undulating currents it created were easily seen in the image.   Also, if I got ANYWHERE near the dewshield, I had to wait 30 seconds for my body heat to clear from the image on the laptop screen.

 

Agree - I think its a very good lens.  In focus, I see a hard Airey Disk with a single diffraction ring.  The single ring does indeed go all the way around, but is slightly brighter on the one side - left side when looking in the eyepiece.

 

I tried to take a image of in-focus star point but had issues with the exposure and gain settings.  Star point was flooding the sensor with light and could not get a sharp image.

 

We just moved from south-central PA to northern suburb of Baltimore.  Much of my test equipment is in storage, my bench with X, Y, Z axis; optical flats etc.   This was a decidedly low tech test with scope on a tripod, artificial star a sufficient distance to not contribute tons of over correction, and camera.  Again trying to determine if the telescope was good/bad so as to give meaningful feedback to the manufacturer.  When I have the time I will do a double pass and maybe put it on an interferometer.  But that requires some setup and auxiliary optics that can take awhile to align.



#17 Kent10

Kent10

    Aurora

  • *****
  • Posts: 4555
  • Joined: 08 May 2012

Posted 10 October 2016 - 01:14 PM

Paul, very nice scope.  Congratulations.  You mentioned the artificial star was placed a sufficient distance to not contribute to over correction.  Fast scopes require even more distance for a valid star test (according to Suiter).  How far was the artificial star?  I tested my TV-140 F5 indoors for collimation but I couldn't get the distance (not even close) for a good star test.  At 92mm maybe you have plenty of room?



#18 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 October 2016 - 01:18 PM

 

Before someone chimes in about using an obstruction, see here:

https://groups.googl...vE/H2bFIbY1WlkJ

 

Hopefully Paul will get his bench up and running again, but even so there are differences between test equipment that are illustrated by the differences between multiple samples of Mr. Rohr's testing and Airylab of the same scope.

 

My guess is that the in focus images of this scope would be quite good, with only the slightest differences from a longer scope visible at high powers in good to excellent seeing.  I am fairly picky (not as much as some of you) but would be happy with this scope for its intended purpose.

 

Derek

 

 

Hi Derek - I'm not a huge fan of the breakout test - not saying it does not work, - but I don't need to perform it each time to determine goodness of the optic.  Reading that SAA thread brought back good memories.  Back in 2000, I was smitten with my Vixen Fluorite.  

 

In any event, I think your right.  Good telescope for its intended purpose.  The lens came with an interferogram.  CFF is has been great to do business with.  Catalin Fus is a nice guy who cares about the product he produces.



#19 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 10 October 2016 - 01:35 PM

Paul, very nice scope.  Congratulations.  You mentioned the artificial star was placed a sufficient distance to not contribute to over correction.  Fast scopes require even more distance for a valid star test (according to Suiter).  How far was the artificial star?  I tested my TV-140 F5 indoors for collimation but I couldn't get the distance (not even close) for a good star test.  At 92mm maybe you have plenty of room?

Yes, plenty of room.  If using a collimated light source as an artificial star, the telescope sees the "star" at infinity.  You can do all sorts of tricks to make this work...  placing a short focal length eye piece in front of star, running a flashlight through a stack of barlows in the focuser of a reflector (light starts at focuser and runs backwards through the telescope) so the artificial start shines out the front of the telescope as a "collimated" point source etc.    See below for additional reading:

 

https://guysmathastr...or-star-tester/

 

Being too close to the star causes the lens to appear grossly overcorrected.   Overcorrection is not the thing that catches my eye.



#20 sg6

sg6

    Fly Me to the Moon

  • *****
  • Posts: 5924
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 10 October 2016 - 03:48 PM

I can see nothing to worry about in those results. Both round and a nice set of concentric circles that are well defined. You are not going to get better, I would be well pleased with an objective that produced that as a result.

Forget the analysis now and go imaging.


  • peleuba likes this

#21 Kent10

Kent10

    Aurora

  • *****
  • Posts: 4555
  • Joined: 08 May 2012

Posted 10 October 2016 - 04:00 PM

 

Paul, very nice scope.  Congratulations.  You mentioned the artificial star was placed a sufficient distance to not contribute to over correction.  Fast scopes require even more distance for a valid star test (according to Suiter).  How far was the artificial star?  I tested my TV-140 F5 indoors for collimation but I couldn't get the distance (not even close) for a good star test.  At 92mm maybe you have plenty of room?

Yes, plenty of room.  If using a collimated light source as an artificial star, the telescope sees the "star" at infinity.  You can do all sorts of tricks to make this work...  placing a short focal length eye piece in front of star, running a flashlight through a stack of barlows in the focuser of a reflector (light starts at focuser and runs backwards through the telescope) so the artificial start shines out the front of the telescope as a "collimated" point source etc.    See below for additional reading:

 

https://guysmathastr...or-star-tester/

 

Being too close to the star causes the lens to appear grossly overcorrected.   Overcorrection is not the thing that catches my eye.

 

Thanks very much, Paul.  Making such an artificial star looks quite involved but maybe one day I'll try it. 



#22 peleuba

peleuba

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1810
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 12 October 2016 - 09:44 AM

 

If one of my high end scopes had the startest shown in your pictures, I would take action to at least cure the decentering/wedge issue, which may even show on axis coma at very high magnifications at the point of best focus.

 

The level of spherical aberration one can live with is a very personal thing. Let me just say I'm a sensitive guy  :lol:

 

Under the stars, all the aberrations in an optical system add up to the final star image a scope produces. Overall, the startest images of your CFF92 do not reach near 1/10th wave P-V quality in my opinion.

 

 

I had a chance to spend some extended time outside with my CFF92 last night.   For part of the session I spent about 90 minutes star testing on Vega soon after dark,  The seeing was quite good for this area and Vega was well placed being overhead, slightly past zenith from my mid-northern latitude.  

 

I stand by what I said previously.  In fact, I think the telescope is better then what my hastily thrown together indoor test showed.  I saw none of the coma/wedge issue on a real star and SA is corrected to a very low level.  Later this week or perhaps next week, I will take some images of a real star and compare to them to my indoor test.  

 

I was struck by how much better the images were then a previous 110 F/5.6 that I had owned.

 

All-in-all this is a very good telescope and compliments my AP130GT my larger Zambuto reflectors nicely.


  • Paulimer, 3 i Guy, doctordub and 4 others like this

#23 gyulaipal

gyulaipal

    Vendor - CFF Telescopes

  • -----
  • Vendors
  • Posts: 7
  • Joined: 01 Dec 2009

Posted 12 October 2016 - 11:33 AM

 

... and SA is corrected to a very low level.  Later this week or perhaps next week, I will take some images of a real star and compare to them to my indoor test.  

 

I was struck by how much better the images were then a previous 110 F/5.6 that I had owned...

As I built the lens of this scope, please allow me to make some comments regarding the indoors test:

 

The optics are corrected for objects in infinite distance. The most reliable test is a real star.

 

Before testing, care must be taken to allow time for the optics to cool/thermalize as perfectly as possible. If the edge of the glass is coolder, spherical undercorrection will be visible. It will be gone after the lens is properly cooled.

 

At green color (532nm wavelength) you should NOT see more than about lambda/50 spherical error (3rd, 5th and 7th order sphericals combined). This is the normal level of spherical correction we aim, and we release lenses that are "worse" than this only when we have a very specific reason (e.g. Strehl is already very high and further figuring would risk making the lens actually worse).

 

On the other hand, due to the (natural) spherochromatism, the spherical correction slightly changes with wavelength. So, testing in red light will show a small spherical undercorrection and using blue light will show the lens spherically overcorrected. This is natural.

 

To see the real spherical correction with minimal "distortion" using an artificial star, you should use a narrow band green light filter (or use a laser based artificial star radiating monochromatic 532nm light) and put the artificial star as far away from the lens, as possible. The VIRTUAL spherical undercorrection (resulting from the finite object distance) in different cases:

  - 5m (16.4 feet) distance results in lambda/4 (!!!) spherical undercorrection

  - 10m (33 feet) distance results in lambda/12 spherical undercorrection

  - 15m (49 feet) distance results in lambda/15 spherical undercorrection.

 

These values are valid for our 92mm APO. Larger lenses need even larger distances.

 

The non-concentric light distribution in the defocused images might be the result of uneven amounts of light reaching different parts of the lens. This looks like coma but the real problem might be the uneven light radiation pattern of the artificial star. Using a real star (where the light flux is surely constant over the whole optical surface) this should NOT be visible.

 

Anyway, I really hope the scope will bring you many happy observing hours... :-)


Edited by gyulaipal, 12 October 2016 - 11:35 AM.

  • Paulimer, peleuba, doctordub and 13 others like this

#24 Erik Bakker

Erik Bakker

    Cosmos

  • *****
  • Moderators
  • Posts: 7992
  • Joined: 10 Aug 2006
  • Loc: Netherlands, Europe

Posted 12 October 2016 - 12:55 PM

Glad that all starts to work out under the (real) stars at infinity Paul  :waytogo:

 

And thanks Pal for chiming in here to clarify and explain!



#25 bb4

bb4

    Mariner 2

  • *****
  • Posts: 268
  • Joined: 16 Apr 2007

Posted 14 October 2016 - 06:57 AM

 

 

If one of my high end scopes had the startest shown in your pictures, I would take action to at least cure the decentering/wedge issue, which may even show on axis coma at very high magnifications at the point of best focus.

 

The level of spherical aberration one can live with is a very personal thing. Let me just say I'm a sensitive guy  :lol:

 

Under the stars, all the aberrations in an optical system add up to the final star image a scope produces. Overall, the startest images of your CFF92 do not reach near 1/10th wave P-V quality in my opinion.

 

 

I had a chance to spend some extended time outside with my CFF92 last night.   For part of the session I spent about 90 minutes star testing on Vega soon after dark,  The seeing was quite good for this area and Vega was well placed being overhead, slightly past zenith from my mid-northern latitude.  

 

I stand by what I said previously.  In fact, I think the telescope is better then what my hastily thrown together indoor test showed.  I saw none of the coma/wedge issue on a real star and SA is corrected to a very low level.  Later this week or perhaps next week, I will take some images of a real star and compare to them to my indoor test.  

 

I was struck by how much better the images were then a previous 110 F/5.6 that I had owned.

 

All-in-all this is a very good telescope and compliments my AP130GT my larger Zambuto reflectors nicely.

 

Hi Paul

Thanks for posting all of this.  I had a CFF 105 and really liked it, but ended up going to a smaller and lighter apo doublet.

 

I do have a question for you which might not be easily answered.  I star test my scopes when they arrive to me new, but I make no claim to be a master at this.  My question is at what point to abberrartion or imperfections that are visible in the star test become easily visible at the EP when the scope is in focus?  The assumption I am making with this question is that some imperfections identified by star tests cannot be (easily) discerned while in focus.

 

I have honestly tried to stay away from becoming better at star testing as I feel I would end up chasing ghosts, but this discussion has intrigued me.

 

Britt


  • doctordub likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics